1. Field of the Invention
The present invention relates to an integrated launch and spacecraft system. More specifically, the present invention relates to a single-stage-to-orbit (SSTO) launch system that is capable of launching a spacecraft into earth orbit and supporting the spacecraft while in orbit.
2. Description of the Related Art
Current spacecraft launch systems fall into one of two classes, recoverable and expendable. Although various recoverable launch systems have been proposed (see, e.g., U.S. Pat. Nos. 3,700,193; 3,576,298; 5,190,246; 5,322,248; and 5,526,999), the only two recoverable launch systems in use are the U.S. space shuttle and the Russian Buran. Both shuttle systems are only partially recoverable in that parts of the booster packages are not retrievable. Expendable launch systems include the European Ariane, the Lockheed Martin Atlas, the McDonald Douglas Delta, the Russian Proton, as well as many others. All existing and previously-developed launch systems, whether reusable or expendable, have been required to use multiple stages to achieve orbital velocity and altitude.
A multistage rocket employs two or more stages that are used in sequence to successively boost a payload into orbit (see, e.g., U.S. Pat. No. 5,529,264, the disclosure of which is incorporated herein by reference). The first stage must provide enough thrust to overcome gravity and to propel the vehicle to an intermediate altitude and velocity. When the first-stage propellants are expended, the tankage and structure used to house the first-stage propellants are jettisoned to reduce the mass of the vehicle. The second stage is then ignited to propel the vehicle to a higher altitude and to a greater velocity. Each stage is expended in sequence and jettisoned, with the final stage achieving the desired orbit and velocity. The final stage separates itself from the payload, which is then free to maneuver and perform its mission unencumbered by the launch vehicle. The reliability of such multistage systems is driven primarily by the reliability of the main thrusters and the staging events.
Multistage systems (of which the payload may be considered the last stage), however, have a number of drawbacks. Perhaps most importantly, multistage systems require one set of systems for the launch phase of their missions, and a second set of systems for the on-orbit phase of their missions. For example, because successive stages are jettisoned to reduce the mass of the vehicle for the succeeding stage, a separate, detachable propulsion system, and a separate detachable structure for supporting that propulsion system, must be provided for the launch phase of the mission in addition to corresponding systems for the on-orbit phase of the mission. Similarly, multistage systems typically have dual control systems and dual power systems that separately support the launch portion of their missions and the on-orbit portion of their missions. Further, multistage systems require complex mechanisms, structures, and electronics to accomplish staging events. The complexity and weight of these mechanisms has a negative impact on system reliability and cost. Accordingly, it is desirable to develop a launch system that eliminates the need for multistaging. Such a SSTO system, however, has not previously been realizable due to the limitations of material and electronics technologies.
Given sufficiently light-weight materials, a SSTO launch vehicle capable of achieving earth orbit could be built. The importance of using lightweight materials for launch systems is illustrated by FIG. 1, which depicts the relationship between overall launch mass and stage mass (tankage and structure) for a SSTO launch vehicle carrying a 10,000 lb payload. Industry experience has shown that the cost of a launch vehicle is roughly proportional to its mass at launch. Thus, as is evident from FIG. 1, a stage mass above roughly 0.1225 results in an infinite launch mass (i.e., a SSTO launch vehicle would not be feasible). However, if a stage mass of roughly 0.12 or less is achievable, then a SSTO launch vehicle becomes practical. The advantages gained by reducing the stage mass is further evident in FIG. 1 by noting that a twenty-percent reduction of stage mass from 0.12 to 0.10 will reduce the launch mass, and the launch cost, by nearly a factor of ten for a SSTO launch vehicle. Similarly, a reduction of stage mass by twenty-five percent from 0.10 to 0.08 will reduce the launch mass and, thus, the launch cost, by nearly an additional factor of two. Accordingly, improvements in technology that can produce relatively small improvements in stage mass fraction will profoundly affect the cost of launch systems, and will greatly enhance the feasibility of a SSTO launch vehicle.
The technologies required to achieve a stage mass of less than or equal to twelve percent are now being developed, and include light-weight graphite composite and aluminum-lithium cryogenic oxidizer/fuel tanks, and simplified light-weight liquid hydrogen/liquid oxygen thrusters. Advancements in other technologies benefitting the development of a SSTO system include miniaturized control valves for handling propellants, and miniaturized electronics for control systems and power management systems.
SSTO systems currently under study, such as the Delta Clipper, are being designed to be recoverable and reusable. Other SSTO systems (or near SSTO) that have been proposed likewise are designed to be recoverable and reusable (see, e.g., U.S. Pat. Nos. 3,756,024 and 5,395,072). However, the need for reentry hardware adds significantly to the technological difficulty and weight of the system. Moreover, such proposed SSTO systems do not eliminate the need for separate subsystems (such as control, propulsion, power, etc.) to support the launch, on-orbit, and reentry phases of their missions. As with multistage systems, the resulting increase in complexity and weight of these proposed systems is likely to adversely affect their reliability and cost effectiveness. Thus, SSTO systems currently under development suffer many of the drawbacks of the multistage systems they are meant to replace.